The technical field of this invention is holography and, in particular, methods and apparatus for controlling the thickness of a holographic recording emulsion during exposure to obtain multi-color image reconstructions.
A hologram is a recording of an interference pattern generated by two overlapping beams of coherent light, one directly from a light source and the other reflected from an object. The recording medium is typically an emulsion formed by a gelatin binder loaded with silver halide microcrystals. When the hologram is displayed by reflection of an illumination beam off the recording medium (or, in the case of transmission-mode holograms, by the transmission of illuminating light through the recording medium), the result is a striking, three-dimensional view of the object.
Considerable efforts have been made during the past twenty years to develop color holograms, particularly reflection-mode color holograms. (Transmission holograms present special problems for the artisan in that the passage of illuminating light through a transmission hologram inherently results in a degree of color smearing, and this problem is greatly compounded when multiple recordings of different colored interference patterns are made.)
In the conventional approach to multi-color reflection holography, beams of coherent light in each of the primary (additive) colors--red, blue and green--are used to record distinct holograms in the photographic plate. In practice registration of the three images is very difficult, particularly for large plates, insofar as the focus and/or magnification of each beam is dependent on its wavelength; good registration requires careful angular adjustment of the beams. The need for additional reference light sources (e.g., a blue light laser and a green light laser, as well as a red light laser) also add a substantial cost to the system. Moreover, the recording medium, e.g., the photographic emulsion, typically has a different sensitivity to different wavelengths, dictating careful control of the exposure time and intensity of each beam as well.
An alternative approach to color holography lies in the use of a single light source and the deliberate variation of the thickness of the emulsion between two or more exposures in order to obtain reconstruction wavelengths that are different from that of the reference light source. These so-called "pseudocolor" reconstructions make multi-color imaging possible with only one color of reference light and thereby permit a single, less expensive laser (e.g., helium-neon or krypton laser generating coherent red light) to be used for recording.
Pseudocolor holographic techniques are disclosed, for example, by Hariharan, Vol 35, Optical Communications, pp. 42-44 (1980). In Hariharan's system, a two-color reflection hologram is obtained by first exposing an emulsion in its normal condition with red light. The emulsion is then soaked in a 3% solution of triethanolamine (TEA) which causes the emulsion to swell. A second exposure is made on the swollen emulsion with the same laser. When the emulsion is subsequently displayed, two distinct reconstructions can be viewed, one at a longer wavelength as a result of the first exposure and the other at a shorter wavelength due to the second exposure. Others have extended this technique using a TEA solution to obtain three-color reflection holograms.
The use of pseudocolor methods has been proposed by the present inventors as a way of rendering three-dimensional computer-generated designs, where computer manipulation of the images and careful design of a holographic optics allow precise registration of the component color images. For further discussion of the application of pseudocolor holography to computer image rendering, see, J. L. Walker, Master Science Thesis, Massachusetts Institute of Technology (February 1987); and S. A. Benton, "The Principles of Reflection Holographic Stereograms," in T. H. Jeong, Ed., Proceedings of the Third International Symposium on Display Holography (1988) herein incorporated by reference.
Nonetheless, current processes for the preparation of pseudocolor holograms continue to suffer serious shortcomings. The use of TEA solutions typically requires manual treatment of the emulsion to insure volumetric loading, and the emulsion surface must then be wiped clean or spun dry to avoid non-uniform swelling. Handling of the plate or film can lead to fogging of the emulsion, and, in any event, removal and replacement of the emulsion in exactly the same position to obtain proper color registration can be problematic.
Moreover, the use of TEA to expand the recording medium can also affect the photographic properties of the emulsion itself insofar as TEA is a sensitizing agent as well as a swelling agent. Thus, TEA tends to degrade previously recorded latent images and enhances the effects of subsequent exposures. To counteract the phenomenon, the exposure time and/or intensity of the exposing light in subsequent recordings must be carefully controlled.
Various alternatives to macromolecular bulk loading of the emulsion with TEA have been proposed. One particularly attractive alternative is the use of water as the primary swelling agent, for example, by immersion of the emulsion in a water/alcohol solution. By varying the water content of the solution, the degree of swelling can be controlled to an extent.
Unfortunately, water-based swelling techniques also have limitations in that gelatin-based emulsions exhibit swelling anomalies when water is introduced into the polymeric matrix. The gels that are most commonly used in emulsions tend to either not swell at all or swell to maximum expansion quickly in the presence of water. This "all or nothing" phenomenon makes it difficult in some instances to achieve an intermediate volumetric state for recording interference patterns that will exhibit a green primary color upon reconstruction.
There exists a need for better holographic color control systems, particularly for recording pseudocolor holograms. A system that could replace conventional bulk loading of an emulsion with a large molecular weight filler, such as TEA, and yet achieve emulsion thickness control without swelling anomalies would satisfy a long-felt need in the art.